1. Field of the Invention
The present invention relates to a confocal optical head, in particular miniature, comprising integrated beam scanning means and a confocal imaging system equipped with said head.
More precisely, the present invention relates to an optical head and a corresponding optical system, in which a field to be imaged in a specimen is scanned point by point by an excitation signal. This type of system makes it possible to obtain, by means of data processing, a confocal image constructed point by point in real time. The confocal character is obtained by means of spatial filtering making it possible to detect a return signal originating solely from the excited point of the specimen taking the same optical path as the excitation signal.
2. Description of the Related Art
One of the intended fields of application is that of high resolution confocal imaging, making it possible to observe and analyze a biological tissue in vivo, in situ, in real time, in particular accessible via the operating channel of an endoscope or integrated into the endoscope. The invention can also be applied to the fields of dermatology or gynaecology requiring less advanced miniaturization of the optical head. It can also be applied to the field of electronics for example for the control of semi-conductors or similar materials.
According to a first type of system, in particular described in the Patent Application WO 00/16151, an image guide is used, constituted by a bundle of flexible optical fibres, comprising at its distal end an optical focusing head intended to come into contact with the specimen to be analyzed. The excitation beam scanning means are situated at the proximal end of the image guide provided for scanning the fibres in turn. The confocal character resides here, in particular, in the fact that the same optical fibre of the guide is used to convey the excitation signal and the return signal emitted. This type of system has the advantage of an optical head simplified from the mechanical point of view and thus essentially comprising optical focusing means which can be miniaturized. On the other hand, it has certain drawbacks, linked to the use of an array of optical fibres, in particular, the problem of sampling the tissue (continuity between the excitation points corresponding to the illumination of a fibre), the problem of injecting the fibres one by one and of the parasitic reflections at the inlet and outlet of the image guide, the sophisticated data processing of the image necessary in order to then correct the pattern of the fibres on the image, etc.
According to another known type of system, the beam scanning means are situated in the optical head at the distal end of an image guide comprising a single flexible optical fibre. The confocal character is obtained here due to the fact that the optical fibre is used for conveying the excitation signal and return signal emitted with an appropriate core diameter of the fibre.
The drawbacks of this type of system are then essentially linked to difficulties of miniaturizing the head, reproducibility and reliability of the mechanical means used for carrying out the scanning of the emergent beam of the optical fibre.
The document U.S. Pat. No. 6,091,067 describes a scanning system in which an optical fibre is fixed to two bimorphic piezoelectric positioners, one of the positioners is placed in the axial direction of the fibre and the other in the direction perpendicular to the optical axis. The positioners, in order to offer an appropriate displacement relative to the field of view, must have a given length. Perpendicular to the axis of the optical fibre, this length constraint in fact leads to an optical head diameter too large for the in vivo applications in situ according to the present invention.
Several documents describe confocal miniature optical heads using micro-mechanical-type micro-mirrors (MEMs).
The Patent Application U.S. 2002/0018276 describes a miniature confocal system using an optical fibre. The light leaving the fibre is reflected on the metallized part of a lens. This light is then reflected on a two-dimensional MEMs micro-mirror surrounding the fibre. The light is then sent towards the specimen via an optical system. The light returning from the specimen follows the reverse path and returns by the fibre which serves for spatial filtering. The system is miniature, being 2 mm in diameter and 2.5 mm in length.
The patents U.S. Pat. No. 6,154,305, U.S. Pat. No. 6,088,145, U.S. Pat. No. 6,007,208, U.S. Pat. No. 5,907,425 and U.S. Pat. No. 5,742,419, describe a confocal head in which the scanning of the field of view is carried out by two electrostatically pivoted MEMs micro-mirrors. The proposed head can be miniaturized but on the other hand offers a 60×60 μm field of view which is too small with respect to the applications according to the invention corresponding to a field measuring 100×100 μm minimum in order to be able to observe, for example, several nuclei which are 5 μm in diameter generally spaced out at intervals of several tens of μm. The number of images per second of 5 to 8 is moreover also insufficient for imagery in real time (requiring a minimum of 10 to 12 images per second in the slowest mode with 640 lines). Moreover also, the field of view is situated parallel to the axis of the optical fibre, which can lead to practical difficulties of use (correct positioning of the probe).
The patents U.S. Pat. No. 6,172,789 and U.S. Pat. No. 6,057,952 describe a confocal optical head with a field of view this time in the axis of the optical fibre and allowing adjustment of the depth of the observation plane. The scanning means include a mobile mirror and a fixed mirror. The mobile mirror has an opening in its centre, and is of MEMs type, mounted so as to be able to pivot at least in one direction. The reflection surfaces of the mobile and fixed mirrors are situated face-to-face. The light leaving the optical fibre passes through the opening in the mobile mirror, then is reflected by the fixed mirror in the direction of the mobile mirror.
The following drawbacks in particular are encountered with such a head construction: significant cost of the mobile mirror with a hole in the centre, and also the scanning speed of such a two-dimensional MEMs mirror which is not sufficient for imagery in real time.
Generally, the change in direction of the optical beam by successive reflections on micro-mirrors leads to optical faults which are complicated to correct, in particular distortion or field curvature.